The present invention relates to electronic circuits, and in particular, to circuits and methods that can be used to receive high frequency input signals.
Traditionally, the overwhelming majority of electronic systems have been connected together using various types of wires or cables. For example, computer systems are typically linked by Ethernet cables, coaxial cables, telephone lines or fiber optic links. Similarly, computer printers, scanners and other peripheral devices such as digital cameras, personal digital assistants or electronic music devices are linked by various types of wires or cables. Other types of electronic devices are similarly connected together using some form of wiring or cabling. However, as electronic systems become more prevalent, there is a growing desire to eliminate the clutter and confusion of the associated wiring.
Wireless technology offers a solution to this growing problem. In a wireless system, electronic devices and systems communicate with each other using electromagnetic signals that propagate through the air. Wireless communication is typically achieved by encoding information in an electronic signal, and then transmitting the signal into the air using an antenna. FIG. 1A illustrates a simplified wireless channel. First, a signal of interest, such as a voice signal or a data signal, is encoded using an encoder 101 in transmitter 110. The encoded signal may then be modulated up to a higher frequency using a modulator 102. After modulation, the encoded voice or data signal is contained within a frequency range centered on the frequency of the modulator as shown in FIG. 1B. The modulator's frequency is sometimes referred to as a “carrier frequency” or “channel frequency,” which may be a very high frequency suited for electromagnetic transmission. Such high frequency signals are referred to as radio frequency (i.e., “RF”) signals because of their historical use in radio transmission systems. A power amplifier 103 receives the modulated signal and drives an antenna 104. Power amplifier 103 must be fast enough to amplify signals at frequencies around the carrier frequency. This means that power amplifier 103 must have the requisite “bandwidth” to process the signal (i.e., the ability to effectively amplify signals across the entire frequency range of interest). The amplified signals drive the antenna, which translates the encoded modulated signal into electromagnetic energy that propagates through the air.
A second antenna 121 in receiver 120 may be used to detect (i.e., sense) the electromagnetic signal. Receiver 120 includes an amplifier 122 that may be used for increasing the amplitude of the signal received on antenna 121. Because electromagnetic transmissions typically decrease in strength rapidly as the distance between transmitter 110 and receiver 120 increases, amplifier 122 must be able to detect signals that may vary across a wide amplitude range as the distance between transmitter 110 and receiver 120 varies. Additionally, when receiver 120 is far away from transmitter 110, the electromagnetic signals can be very small (e.g., microwatts). Thus, receiver 120 must be able to amplify very small signals without introducing noise. Moreover, similar to power amplifier 103, amplifier 122 must be fast enough to amplify signals at frequencies around the carrier frequency. After the encoded modulated signals is amplified, demodulator 123 may be used to down-convert the signal's frequency, and decoder 124 may be used to extract the original voice or data signal. Since communication is typically a two-way process, most wireless systems will include both a transmitter 110 and a receiver 120 coupled to an antenna.
While wireless technology has successfully been used to implement such systems as radio, analog television and cellular telephone systems, it has one major limitation—data rate. Over the last decade, the number of electronic devices has increased drastically. Furthermore, the information capacity of each device has similarly increased. Thus, the amount of data that must be communicated between modern electronic devices has become enormous. Wired systems are advantageous because communication channels between devices are confined to the wires that connect them. These separate wires all represent separate communication medium. Wireless devices, on the other hand, all share the same communication medium—the airwaves. Consequently, wireless devices typically communicate with each other across predefined frequency ranges (i.e., channels).
For wireless devices to replace existing wired connections, more effective frequency allocations are required to handle the large amount of information that is transmitted and received between various types of electronic systems. This is typically achieved by moving to much higher frequencies than have been used in the past. In particular, while frequencies in the megahertz range (“MHz”) have been commonplace in the past, frequencies in the gigahertz range (e.g., above 2 GHz) may be used for larger data rates. For example, a 10 GHz range may be divided up into a plurality of individual 500 MHz wide subranges (i.e., subbands), each of which may be used as a channel to carry information. Thus, for a wireless system to maximize the data rate, it should be able to operate effectively across a wide range of frequencies (e.g., 3 GHz-10 GHz). Consequently, what is needed is a receiver that can operate across a wide range so that a single system can receive multiple frequency channels across the gigahertz range.
Contemporary receivers suffer from a variety of shortcomings. FIG. 2A illustrates a simplified prior art receiver amplifier 200. Amplifier 200 includes an MOS transistor 201 having a gate coupled through an inductor 204 (“Lg”) to receive an RF input signal (“Vin”), a source coupled through an inductor 205 (“Ls”) to ground and a drain coupled to a capacitor (“C”) 202 and an inductor (“L”) 203. Capacitor 202 is coupled to ground and inductor 203 is coupled to a supply (“Vcc”). Amplifier 200 is effective at very high frequencies because values for capacitor 202 and inductors 203-205 may be selected so that LC combinations resonate at a frequency of interest (i.e., the center frequency of the circuit). For example, L and C may be selected so that the output LC combination provides the necessary output impedance at high frequency for proper amplification of the signal.
In addition to achieving amplification at high frequencies, it is also necessary to achieve proper input matching. For example, when an antenna is coupled to the input of an amplifier for receiving an RF signal, the input impedance of the amplifier must be matched to the antenna. This is sometimes accomplished by placing a resistor at the amplifier input. However, resistors generate noise, and noise at the input of a receiver amplifier is very problematic because the noise level may be on the same order of magnitude as the incoming RF signal. Thus, using resistors at the input of an amplifier can make it difficult to distinguish signal from noise.
FIG. 2B is an equivalent input circuit for amplifier 200 that illustrates the advantageous noise properties of this architecture. The input impedance circuit of the amplifier includes series connected inductor 204 (“Lg”), MOS gate capacitance 206 (“Cg”), inductor 205 (“Ls”) and an equivalent resistance 207 (“R1”). One advantage to amplifier 200 is that the values of inductors 204 (“Lg”) and 205 (“Ls”) may be sized so that the inductance and capacitance of the input circuit resonates at frequencies of interest (i.e., Lg, Cg and Ls are a short circuit at predetermined frequencies). Thus, the input impedance of the amplifier is set by the value of R1 near the resonant frequency. Since R1 is an equivalent resistance of the MOS transistor (e.g., R1=(gmLs)/Cgs), and not a physical resistor, the input impedance of the amplifier may be matched to the antenna without generating the noise typically associated with a physical resistor.
However, while such circuits have good noise performance and gain for particular frequencies, they are not effective across wide frequency ranges because as the signal frequency moves above or below the resonant frequency, the capacitance or inductance begins to dominate quickly. Consequently, amplifier 200 will not be able to provide amplification or input matching for input signals across a wide frequency range.
FIG. 3 illustrates a simplified diagram of a prior art approach to designing a high frequency amplifier that operates across a wide range of frequencies. Amplifier 300 includes a plurality of narrowband amplifiers 301-304 that are each optimized to operate in different narrow frequency ranges centered at a specific center frequency. The input of each amplifier is coupled through a corresponding switch S1-Sn to an antenna 310. When a specific frequency range is desired, the system closes the switch connecting the antenna to the particular amplifier optimized to operate across the desired frequency range. One problem with this approach is that information cannot be received on different frequency ranges unless the switches are reconfigured. Additionally, each switch introduces problematic parasitic capacitances. Furthermore, this approach is very expensive because of the additional components and integrated circuit die area required to implement multiple channels.
FIG. 4 illustrates a simplified diagram of another prior art amplifier circuit technique. One way of extending the frequency range of amplifier 400 is to provide resistive feedback. Amplifier 400 includes an MOS transistor 401 having gate coupled to receive an RF input signal (“Vin”), a source coupled to ground and a drain coupled to the output (“Vout”). A feedback resistor (“R”) 402 is coupled between the drain output and source input. Resistor 402 may be set to provide wide band input matching and gain performance. However, while the frequency range of input matching and amplification may be extended as compared to amplifier 200, such techniques suffer from a variety of problems. First, resistors generate noise, and noise at the input will be amplified. Moreover, a portion of the noise at the output is also fed back and amplified. Thus, amplifiers using resistive feedback typically have a very poor noise figure (“NF”). Furthermore, feedback architectures typically have very high power consumption, which is also undesirable.
FIG. 5 illustrates a simplified diagram of another prior art amplifier circuit. Amplifier 500 illustrates a common gate approach. Amplifier 500 is advantageous because the source impedance of MOS transistor 501 may be used for impedance matching, thereby eliminating the need for a physical resistor for input matching. For example, the input impedance of M1 may be matched to the antenna by setting the device properties of M1. As a result, no physical resistor is necessary and the noise figure of the receiver is improved. However, one major problem with amplifier 500 is that common gate amplifiers have very low gain compared to other architectures.
Thus, there is a need for a high frequency receiver amplifier with improved noise performance that simultaneously provides improved gain and input matching across an ultra-wide bandwidth. The present invention solves these and other problems by providing high frequency wireless circuits and methods as describe below.